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While it has been known for half a century that a plant’s pores, called stoma, can open at varying rates depending on the concentration of carbon dioxide in the air, scientists did not understand how the process worked until now, said Julian Schroeder, the study’s lead author and a professor of biology at the University of California, San Diego. Dr. Schroeder and his colleagues report that they identified the specific sensors in plants that detect carbon dioxide and prompt a plant’s pores to open and breathe.

The tighter a plant can keep its pores, the less water it loses and the less water it requires to grow. And since carbon dioxide levels are 40 percent greater than in preindustrial times, it would seem that plants could now get plenty of carbon dioxide with out losing too much water  an odd and perhaps beneficial consequence of the accumulation of greenhouse gases.

The water saving strategy does not work for all plants, and the problem is how well they detect carbon dioxide. Plants with sensitive sensors can save water. But other plants with weaker carbon dioxide receptors are not able to detect the increased levels as well and miss the opportunity to conserve water, Dr. Schroeder said.

His laboratory found that by adding extra copies of the receptor proteins, called carbonic anhydrases, pores tightened, water efficiency increased and the plants still got enough carbon dioxide. Dr. Schroeder said genetically manipulating certain plants could provide farmers with more efficient crops.

“It could be useful in California for instance,” he said. “What we see is that plants can grow, but they need less water.” In California, 79 percent of water diverted from streams and rivers and pumped from the ground is used in agriculture, according to the State Department of Water Resources.

Wolf Frommer, a Stanford biologist who was not involved in the study, said that while humans found ways to battle growing carbon dioxide emissions and manage a diminishing water supply, the study offered one way to help plants cope with environmental changes.

“With increasing global warming, at least we are able to provide potential solutions,” Dr. Frommer said. “If we understand how this works, we can grow plants using transgenic approaches.”

There is one problem, though, with helping plants conserve water. When plants release moisture through their pores, it cools their leaves, in the same way sweating cools humans. This prevents them from drying out, particularly in desert regions.

But Dr. Frommer explained that bioengineering was a slow tweaking process. He said there might be another yet undiscovered gene that when manipulated could prompt a plant to open pores when its leaves were dry.

“We have to think less in terms of turning one gene up or down, but have an integrated solution,” he said. “It’s what evolution has been doing for billions of years. We have to do it faster to feed an increasing population.”